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Search Results: 1 - 10 of 404175 matches for " D. L. Wu "
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Similarity of climate control on base flow and perennial stream density in the Budyko framework
D. Wang ,L. Wu
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 2013,
Abstract: Connection between perennial stream and base flow at the mean annual scale exists since one of the hydrologic functions of perennial stream is to deliver runoff even in low flow seasons. The partitioning of precipitation into runoff and evaporation at the mean annual scale, on the first order, is captured by the ratio of potential evaporation to precipitation (EP/P called climate aridity index) based on Budyko hypothesis. Perennial stream density (DP), which is obtained from the high resolution National Hydrography Dataset, for 185 watersheds declines monotonically with climate aridity index, and an inversely proportional function is proposed to model the relationship between DP and EP/P. The monotonic trend of perennial stream density reconciles with the Abrahams curve since perennial stream density is only a small portion of the total drainage density. The correlation coefficient between the ratio of base flow to precipitation (Qb/P), which follows a complementary Budyko type curve and perennial stream density is found to be 0.74. The similarity between Qb/P and DP reveals the co-evolution between water balance and perennial stream network.
Similarity between runoff coefficient and perennial stream density in the Budyko framework
D. Wang,L. Wu
Hydrology and Earth System Sciences Discussions , 2012, DOI: 10.5194/hessd-9-7571-2012
Abstract: Streams are categorized into perennial and temporal streams based on flow durations. Perennial stream is the basic network, and temporal stream (ephemeral or intermittent) is the expanded network. Connection between perennial stream and runoff generation at the mean annual scale exists since one of the hydrologic functions of perennial stream is to deliver runoff. The partitioning of precipitation into runoff and evaporation at the mean annual scale, on the first order, is represented by the Budyko hypothesis which quantifies the ratio of evaporation to precipitation (E/P) as a function of climate aridity index (Ep/P, ratio of potential evaporation to precipitation). In this paper, it is hypothesized that similarity exists between perennial stream density (Dp) and runoff coefficient (Q/P) as a function of climate aridity index, i.e., DpDp* (EpP) and QP (EpP) where Dp* is a scaling factor and Q is mean annual runoff. To test the hypothesis, perennial stream densities for 185 watersheds in the United States are computed based on the high resolution national hydrography dataset (NHD). The similarity between perennial stream density and runoff coefficient is promising based on the case study watersheds. As a potential application for macroscale hydrological modeling, perennial stream density in ungauged basin can be predicted based on climate aridity index using the complementary Budyko curve.
Imaging gravity waves in lower stratospheric AMSU-A radiances, Part 1: Simple forward model
S. D. Eckermann,D. L. Wu
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2006,
Abstract: Using a simplified model of in-orbit radiance acquisition by the Advanced Microwave Sounding Unit (AMSU-A), we derive three-dimensional temperature weighting functions for Channel 9 measurements (peaking at ~60–90 hPa) at all 30 cross-track beam positions and use them to investigate the sensitivity of these radiances to gravity waves. The vertical widths of the weighting functions limit detection to waves with vertical wavelengths of 10 km, with slightly better vertical wavelength sensitivity at the outermost scan angles due to the limb effect. Fourier Transforms of two-dimensional cross-track weighting functions reveal optimal sensitivity to cross-track wavelengths at the near-nadir scan angles, where horizontal measurement footprints are smallest. This sensitivity is greater for the AMSU-A on the Aqua satellite than for the identical instruments on the NOAA meteorological satellites, due to a lower orbit altitude and thus smaller horizontal footprints from antenna spreading. Small cross-track asymmetries in the radiance response to gravity waves are found that peak at the mid-range scan angles, with more symmetric responses at near-nadir and far off-nadir scan angles. Three-dimensional simulations show gravity wave oscillations imaged in horizontal AMSU-A radiance maps swept out by the scan pattern and satellite motion. A distorting curvature is added to imaged wave phase lines due to vertical variations in weighting function peaks with cross-track scan angle. This wave distortion is analogous to the well-known "limb darkening" and "limb brightening" of microwave radiances acquired from purely vertical background temperature profiles by cross-track scanners. Waves propagating along track are more visible in these images at the outermost scan angles than those propagating cross track, due to oversampling and narrower widths of the horizontal measurement footprints in the along track direction. Based on nominal noise floors and representative lower stratospheric wave temperature amplitudes, our modeling indicates that Channel 9 AMSU-A radiances can resolve and horizontally image gravity waves with horizontal wavelengths of 150 km and vertical wavelengths of 10 km.
Epidural Anesthesia-Analgesia and Patient Outcomes: A Perspective
Christopher L. Wu,Jamie D. Murphy
Advances in Anesthesiology , 2014, DOI: 10.1155/2014/948164
Abstract: The use of perioperative epidural anesthesia-analgesia may confer many benefits including superior postoperative analgesia, decreased morbidity, and improvement in patient-centered outcomes. We will review our group’s work on perioperative epidural anesthesia-analgesia on patient outcomes and discuss future research in this area 1. Introduction The use of perioperative epidural anesthesia-analgesia provides superior postoperative analgesia [1–3] versus conventional opioids, is associated with beneficial physiologic effects [4], and has been shown to improve patient-centered outcomes [5]. However, the benefits of perioperative epidural anesthesia-analgesia are not definitive in part due to some of the methodological issues present in available studies. We will review our group’s work on perioperative epidural anesthesia-analgesia on patient outcomes which includes mortality, major morbidity, and patient centered outcomes but not technical failures or complications. We will also discuss future research in this area. 2. Effect on Mortality The overall effect of epidural anesthesia-analgesia on perioperative mortality is controversial. Because of the relatively rare incidence of this endpoint, one option is to utilize a large database which would be able to capture rare outcomes such as death. We have used a 5% nationally random sample of Medicare beneficiaries (1997–2001) to examine the association between the presence or absence of postoperative epidural analgesia and perioperative (7- and 30-day) mortality [6–10]. In our initial analysis [6], we examined several surgical procedures (based on International Statistical Classification of Diseases, Ninth Revision (ICD-9) codes) and divided patients into 2 groups, depending on the presence or absence of postoperative epidural analgesia. Mortality at 7 and 30 days after surgery was examined and multivariate regression analyses incorporating race, gender, age, comorbidities, hospital size, hospital teaching status, and hospital technology status were undertaken. Our first publication using the Medicare claims data found that the presence of epidural analgesia was associated with significantly lower odds of death at 7 days (odds ratio (OR) = 0.52; 95% confidence interval (CI): 0.38–0.73; ) and 30 days (OR = 0.74; 95% CI: 0.63–0.89; ) postoperatively, thus suggesting that postoperative epidural analgesia may contribute to lower odds of death after surgery [6]. Subsequent analyses of the Medicare claims data also revealed that the presence of postoperative epidural analgesia may be associated with lower
Ballistic transport: A view from the quantum theory of motion
Hua Wu,D. W. L. Sprung
Physics , 1994, DOI: 10.1016/0375-9601(94)91231-9
Abstract: Ballistic transport of electrons through a quantum wire with a constriction is studied in terms of Bohm's interpretation of quantum mechanics, in which the concept of a particle orbit is permitted. The classical bouncing ball trajectories, which justify the name ``ballistic transport'', are established in the large wave number limit. The formation and the vital role of quantum vortices is investigated.
Power of Anisotropic Exchange Interactions: Universality and Efficient Codes for Quantum Computing
L. -A. Wu,D. A. Lidar
Physics , 2001, DOI: 10.1103/PhysRevA.65.042318
Abstract: We study the quantum computational power of a generic class of anisotropic solid state Hamiltonians. A universal set of encoded logic operations are found which do away with difficult-to-implement single-qubit gates in a number of quantum computer proposals, e.g., quantum dots and donor atom spins with anisotropic exchange coupling, quantum Hall systems, and electrons floating on helium.We show how to make the corresponding Hamiltonians universal by encoding one qubit into two physical qubits, and by controlling nearest neighbor interactions.
Universal Quantum Computation using Exchange Interactions and Teleportation of Single-Qubit Operations
L. -A. Wu,D. A. Lidar
Physics , 2002, DOI: 10.1103/PhysRevA.67.050303
Abstract: We show how to construct a universal set of quantum logic gates using control over exchange interactions and single- and two-spin measurements only. Single-spin unitary operations are teleported instead of being executed directly, thus eliminating a major difficulty in the construction of several of the most promising proposals for solid-state quantum computation, such as spin-coupled quantum dots, donor-atom nuclear spins in silicon, and electrons on helium. Contrary to previous proposals dealing with this difficulty, our scheme requires no encoding redundancy. We also discuss an application to superconducting phase qubits.
Encoded Recoupling and Decoupling: An Alternative to Quantum Error Correcting Codes, Applied to Trapped Ion Quantum Computation
D. A. Lidar,L. -A Wu
Physics , 2002, DOI: 10.1103/PhysRevA.67.032313
Abstract: A recently developed theory for eliminating decoherence and design constraints in quantum computers, ``encoded recoupling and decoupling'', is shown to be fully compatible with a promising proposal for an architecture enabling scalable ion-trap quantum computation [D. Kielpinski et al., Nature 417, 709 (2002)]. Logical qubits are encoded into pairs of ions. Logic gates are implemented using the Sorensen-Molmer (SM) scheme applied to pairs of ions at a time. The encoding offers continuous protection against collective dephasing. Decoupling pulses, that are also implemented using the SM scheme directly to the encoded qubits, are capable of further reducing various other sources of qubit decoherence, such as due to differential dephasing and due to decohered vibrational modes. The feasibility of using the relatively slow SM pulses in a decoupling scheme quenching the latter source of decoherence follows from the observed 1/f spectrum of the vibrational bath.
Dressed Qubits
L. -A. Wu,D. A. Lidar
Physics , 2003, DOI: 10.1103/PhysRevLett.91.097904
Abstract: Inherent gate errors can arise in quantum computation when the actual system Hamiltonian or Hilbert space deviates from the desired one. Two important examples we address are spin-coupled quantum dots in the presence of spin-orbit perturbations to the Heisenberg exchange interaction, and off-resonant transitions of a qubit embedded in a multilevel Hilbert space. We propose a ``dressed qubit'' transformation for dealing with such inherent errors. Unlike quantum error correction, the dressed qubits method does not require additional operations or encoding redundancy, is insenstitive to error magnitude, and imposes no new experimental constraints.
Creating Decoherence-Free Subspaces with Strong and Fast Pulses
L. -A. Wu,D. A. Lidar
Physics , 2001, DOI: 10.1103/PhysRevLett.88.207902
Abstract: A decoherence-free subspace (DFS) isolates quantum information from deleterious environmental interactions. We give explicit sequences of strong and fast (``bang-bang'', BB) pulses that create the conditions allowing for the existence of DFSs that support scalable, universal quantum computation. One such example is the creation of the conditions for collective decoherence, wherein all system particles are coupled in an identical manner to their environment. The BB pulses needed for this are generated using only the Heisenberg exchange interaction. In conjunction with previous results, this shows that Heisenberg exchange is all by itself an enabler of universal fault tolerant quantum computation on DFSs.
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